X-Mailer
Optional email header identifying the sending software — visible to recipients, sometimes used as a spam signal.
X-Mailer is an optional email header that identifies the software used to compose or send the message — values like "Microsoft Outlook 16.0" or "Mailchimp Mailer". The header is visible to anyone inspecting the raw message and is sometimes used (along with other signals) as a spam-classification input.
X-Mailer values can be a small reputation signal. Bulk senders identified by an ESP's X-Mailer get different treatment than personal senders identified by a desktop client. Spam-filtering services keep heuristics linking certain X-Mailer strings to higher spam probability.
For most senders, X-Mailer is not a lever to optimize — it's set automatically by your ESP or mail client. Spoofing or removing it is rarely useful and sometimes backfires (missing X-Mailer can itself be a slight spam signal). Keep it default and put optimization effort elsewhere.